There have been various technological approaches to produce a writing device as a replacement of paper and pencil or chalk on slate. The best known examples are toys. The ETCH-A-SKETCH™, introduced in the 1960s, is one such device. In this device, a movable stylus removes a powder material from inside a screen to make a dark line. The image is erased by turning the device upside down and shaking it to smooth out the surface. Another famous example is the MAGNA DOODLE™, which is a magnetophoretic device in which a stylus with a magnet on the tip is used as the pen to draw a line. The device is erased with a thin long magnet behind the screen. Over 40 million of these devices have been reportedly sold.
Other LCD writing devices have also been proposed. U.S. Pat. No. 4,525,032 to Hilsum is one such example where cholesteric or a smectic liquid crystal is used to provide a semi-permanent record of the path traced by a stylus on a display and used as a re-usable writing pad. According to Hilsum a layer of a liquid crystal material is contained between two substrates. A stylus having a tip contacts the front substrate and changes the state of selected areas of the liquid crystal layer at positions adjacent the pen tip to provide observable information corresponding to the pen movement. The pen may have a pointed tip, a heated tip, a light emitting tip, or a tip connected to a high voltage high impedance source. At least one substrate of the display can be deformable, thin, or flexible so that the liquid crystal layer may be changed from one state to another by localized application of pressure, heat, light, electrostatic charge, or an electric field. The resultant image on the display is erased by deformation of the layer, e.g. flexing, heating and cooling, or by an electrical field.
A practical problem with the Hilsum device is erasing the image. It is slow and inconvenient to heat or flex the device to erase the image. Hilsum discloses an electronic means of erasure using a special cholesteric liquid crystal in which the frequency of an AC field is applied to the stylus or electrodes. The frequency is changed to enable a writing state or an erasure state. However, this is not without problems in that crossover frequency between writing and erasing is strongly temperature dependent and the frequencies as well as the voltages are very high, consuming a lot of power causing very limited battery lifetime.
A considerable improvement was made with the discovery of bistable cholesteric liquid crystals (see U.S. Pat. No. 5,453,863). Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. These materials possess a helical structure in which the liquid crystal (LC) director twists around a helical axis. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. They can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth by controlling the pitch of the helical twist through the concentration of chiral dopants and the birefringence of the nematic host, respectively. If the incident light is not polarized, it will be decomposed into two circular polarized components with opposite handedness and one of the components reflected.
The cholesteric material is typically electrically switched to either one of two stable textures; planar or focal conic as described, for example, in the U.S. Pat. No. 5,453,863. In the planar texture, the director of the LC (direction of the long axis of the molecule) is uniformly parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high; thus the helical domain size becomes small and randomized in orientation such that it is just weakly scattering and does not reflect impinging light (i.e., it is substantially transparent to incident light). Once the defect structures are created, they are topologically stable and cannot be removed unless by some external force such as an electric field or melting the material out of the liquid crystalline phase to the isotropic. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. These bistable structures can be electronically switched between each other at rapid rates (on the order of milliseconds). Gray scale is also available within a single pixel through various switching schemes in order to adjust the density of reflective helical domains that are oriented perpendicular to the substrates (planar texture) to the randomized forward scattering domains (focal conic texture).
Bistable cholesteric liquid crystal displays have several electronic drive features that other bistable reflective technologies do not. For addressing a matrix of many pixels in a display, the characteristic of a voltage threshold is used. A threshold is used for multiplexing a row/column matrix without the need of an expensive active matrix (transistor at each pixel). Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology. Gray scale capability allows stacked RGB, high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated.
In a cholesteric liquid crystal display, the liquid crystal is typically sandwiched between two substrates that are spaced to a particular gap. The substrates can be either glass or polymer. The bottom substrate is painted with a light absorbing (black or colored) background. The cell gap is usually set by polymer or glass spacers that are either cylindrical or spherical in shape. In most cholesteric liquid crystal displays, the cell gap is not intentionally changed. If one presses on the top substrate of the cholesteric LCD, the liquid crystal can be displaced (since fluids are not very compressible) and induced to flow radially out of the area. Of principle interest is that when the focal conic texture of the cholesteric liquid crystal is induced to flow, the resulting texture is the planar state. The reflective planar state contrasts well to the dark focal conic background. This is a principle behind U.S. Pat. No. 6,104,448 “Pressure Sensitive Liquid Crystalline Light Modulating Device and Material,” incorporated herein by reference in its entirety, which discloses that application of a mechanical stress to the liquid crystalline light modulating material changes an initial light scattering focal conic texture to the light reflecting planar texture. The U.S. Pat. No. 6,104,448 discloses a polymer network that is soluble with the chiral nematic liquid crystal and phase separates to form separated polymer domains that stabilize the thickness of the cell structure.
In U.S. Pat. No. 6,104,448, an image can be written on the device with an untethered stylus or fingernail. The entire image is erased with the push of a button that applies a low voltage DC pulse to the cholesteric device. Other advantages of bistable cholesteric materials is that an image created on the writing pad display does not degrade with time and lasts indefinitely without application of an electric field, until erased. The time for erasing the entire image is less than a second, making the bistable cholesteric liquid crystal display a practical device for a writing pad. However, the device of the U.S. Pat. No. 6,104,448 completely erases the entire writing surface and does not erase selected portions of the image.
eWriter with Select Erase by Kent Displays Inc.
U.S. Pat. No. 8,139,039 discloses an LCD writing tablet or eWriter using bistable cholesteric liquid crystal technology that can be selectively erased. The U.S. Pat. No. 8,139,039 patent is incorporated herein by reference in its entirety and this disclosure includes the following text and figures excerpted from that patent. The U.S. Pat. No. 8,139,039 patent describes writing tablet devices and methods for selectively erasing the tablet using an untethered stylus. The selectively erasable writing tablet works in different Modes: Mode A, B and multimode (in the case of a multilayer device). In Mode A, the pressure of an instrument (e.g., an untethered pointed stylus) creates a transparent focal conic texture line on a colored planar texture background where all layers are switched to the planar texture. In Mode B, which is disclosed in prior art such as the U.S. Pat. No. 6,104,448, the stylus creates a colored planar texture line on a transparent focal conic background where all layers are switched to the focal conic texture. In multimode (for a multilayer device), the pressure of an instrument creates a colored planar texture line or a focal conic texture line on a user selected colored texture background where the layers in the background can be either planar, focal conic or gray scale. In Modes A and B and multimode a gray scale reflective texture can be employed for the background and/or the written portion that has a reflectance between a maximum level of reflectance of the planar texture and a minimum level of reflectance of the focal conic texture. Gray scale can be formed by varying voltage or pressure as discussed in U.S. Pat. No. 8,228,301, which is incorporated herein by reference in its entirety. Contrast in the writing pad is created between the transparent focal conic texture which shows the light absorbing back-layer color or black and the reflective (color) planar and gray scale textures.
Turning now to a more specific discussion of the prior art LCD writing device by Kent Displays Inc. having select erase capability, much of which is extracted from the U.S. Pat. No. 8,139,039 patent owned by Applicant, a bistable cholesteric LCD writing device or eWriter is illustrated in FIG. 1 and is incorporated herein by reference for all of its features, modified as described in the Detailed Description below. The LCD eWriter includes substrates 11 of flexible material, polymeric material or plastic being preferred. The inner surfaces of the substrates are coated with transparent electrically conductive layers or electrodes 12 (preferably unpatterned but possibly patterned) from materials such as indium tin oxide (ITO) or a conducting polymer such as PEDOT. Sandwiched between the adjacent electrodes is a bistable cholesteric liquid crystal material 23. The droplets of liquid crystal are not encapsulated and are thus unconfined with interconnecting droplets. The dispersions may be of the type prepared by polymerization induced phase separation (PIPS) as is known in the art. The conductive electrodes 12 are connected with electrical interconnects 14 to electronic circuit 25 that provides suitable voltages to the conductive electrodes 12, usually in the form of a voltage pulse, in order for pressure of the stylus to create an image.
Planar Writing on a Focal Conic Background, Mode B:
In this embodiment in which the liquid crystal is initially in the focal conic texture, while not wanting to be bound by theory, flow of the liquid crystal forms the planar texture, under the pressure of a stylus without application of a voltage, in order to write. The image is created by the planar writing or drawing contrasting against the focal conic background. Once an image has been made on the eWriter it can be electronically erased. The conductive electrodes 12 are connected with electrical interconnects 14 to electronic circuit 25 that provides suitable voltages to the conductive electrodes 12, usually in the form of a pulse, in order to selectively or fully erase an image. The circuit 25 is first switched to operate in Mode B (planar writing on a focal conic background) and then the user can select the various functions; write, select erase, or full erase.
Mode B Full Erase:
The procedure of writing an image on the inventive cell in Mode B is to first fully erase all previous images by selecting the “Mode B full erase” function of the circuit 25. This applies an erasing voltage of value Vf indicated by vertical line 37 of FIG. 2 to drive the entire cell initially to the focal conic texture. The value of the voltage or magnitude of a voltage pulse to do this is well known in the art of a bistable cholesteric liquid crystal displays; e.g., U.S. Pat. Nos. 5,453,863 and 5,691,795. This erases the writing tablet to the mostly transmissive focal conic texture so that the background (black or a backcolor) of the writing tablet is due to revealing the back layer 19 of FIG. 1.
Mode B Write:
In order to write an image using stylus 16 in Mode B, one activates the “Mode B write” function on the circuit 25 such that no voltage is applied to the writing tablet. In Mode B, which is disclosed in prior art such as the U.S. Pat. No. 6,104,448, the stylus creates a colored planar texture line on a transparent focal conic background. Contrast in the LCD writing device is created between the transparent focal conic texture which shows the back-coating 19 (typically black) and the reflective (color) planar texture of the cholesteric liquid crystal. An example of a selectively erasable writing tablet cell operating in Mode B is photographed in FIG. 3. The write circuit 25 can fully erase any image by selecting the “Mode B full erase” function to provide a focal conic erase voltage Vf 37 to the tablet. It is seen by FIG. 2 that a voltage Vf 37 will drive the entire cell to the focal conic texture, erasing the planar writing. The voltage may be an AC voltage pulse or a sequence of pulses, preferably bipolar pulses.
Mode B Select Erase:
In order to selectively erase an image using stylus 16, one activates the “Mode B select erase” function on circuit 25 such that a voltage Vw 36 is applied to the writing tablet. The voltage is applied for the duration of the pressure applied by the stylus; preferably a continuous AC voltage or a sequence of bipolar pulses. The voltage is applied to the patterned or unpatterned electrodes so that the entire display area of liquid crystal seen by the user of the writing tablet, has an electric field applied to it. It is seen by curve 32 in FIG. 2 that a voltage Vw 36 with applied pressure will selectively erase by driving that area of the cell to the focal conic texture; that is, areas of the cell under the stylus where pressure is applied and cell gap is reduced. The planar texture in that area is driven to the focal conic as illustrated by the dashed curve 32. In the remainder of the cell where the cell gap is not disturbed, even though the electric field has been applied there, writing on the cell in which the material is in the planar texture remains planar as indicated by the solid curve 31, leaving a focal conic background for the planar written portion. In other words, the planar writing where the cell gap is undisturbed remains in the planar texture as shown where line Vw intersects the solid line 31 (undepressed cell gap) while the voltage Vw is applied to the electrodes and is not converted to the focal conic texture. The regions of the writing tablet that are selectively erased to the focal conic texture blend with the switched focal conic texture of the background such that the erased line does not contrast, as is evident in FIG. 4. Note that the planar line of FIG. 3 underneath the planar text “Select Erase” was retraced while the tablet was in select erase mode and is barely visible in FIG. 4.
Turning away now from the excerpt of the U.S. Pat. No. 8,139,039, Mode B select erasing employs application of suitable voltages to the writing tablet electrodes during application of pressure to the writing tablet. However, the voltage levels that are suitable for a given device at any particular time are dependent on a number of factors. Without wanting to be bound by theory, some of these factors may include liquid crystal formulations and processing, device to device variations as a result of manufacturing tolerances, temperature, device aging, and device history such as amount of ultraviolet light and heat exposure.
It would be desirable to have an electronic LCD eWriter device in which the voltage waveforms applied to the electrodes during Mode B select erasing are adjustable to address differences between devices that result from liquid crystal formulations and processing, manufacturing tolerances, aging, and history, as well as to support a wide range of operating conditions.